It is known to provide an isolator on an engine crankshaft or on a belt-driven accessory, such as an MGU (motor generator unit) or an alternator, that is driven by a belt from the crankshaft of an engine in a vehicle. As is known, the crankshaft undergoes cycles of accelerations and decelerations associated with the firing of the cylinders in the engine. The isolator permits these accelerations and decelerations to occur with reduced effect on the speed of the belt. A problem with some isolators is that they do not provide ‘decoupling’ which permits overrunning of the belt and the pulley relative to the crankshaft of the engine. Some decouplers have been proposed which include powered clutches to provide decoupling and which also permit BAS (belt/alternator start) capability for the engine when desired. However, such decouplers can be complex. It would beneficial to provide a decoupler that at least partially addresses this problem.